1. Field of the Invention
The present invention relates to a paste-type electrode for an alkali secondary cell, more specifically to that having a improved three-dimensional substrate.
2. Description of the Related Art
There are several types of electrode used in an alkali secondary cell such as a sinter type, paste type, and pocket type. Of all the types, the paste type electrode is most widely used, because the amount of the active material occupying the electrode can be increased, the electric capacity density can be enhanced, and the production cost can be reduced.
The paste type nickel electrode can be prepared, for example, by the following manner. Nickel hydroxide powder, an appropriate binder, and water are mixed to make paste. The paste is applied on a two-dimensional substrate such as expanded metal, porous metal, or nickel net to fill the empty spaces of the substrate. After, the substrate filled the paste is dried and pressed.
However, the paste-type nickel electrode manufactured as above entails a problem arising from the constitution of the electrode. In the electrode of this type, the distance between the particles of nickel used as the activation substance and the metal matrix constituting the two-dimensional electrode serving as a collector, is tens of times longer than that of the conventional sinter-type nickel electrode. Consequently, the paste-type electrode exhibits a conductivity poorer than that of the conventional electrode, creating the problem of a low utilization of the nickel hydroxide, as compared to that of the conventional sinter-type nickel electrode. The difference between these types of electrodes in utilization is prominent especially in the case of a large current discharge.
In consideration of the above-described problem, Published Unexamined Japanese Patent Application (PUJPA) No. 50-32450 discloses a method for manufacturing a felt-like metal porous member made of hollow metal fibers comprising steps of coating the surface of organic fibers with alkali-proof metal, and baking the organic fiber. This metal porous member is used as a three-dimensional substrate. In a three-dimensional substrate including the above-described felt-like metal porous member, the distribution of the hollow metal fibers can be made uniform, thereby reducing the irregularity of the electrode characteristics. Furthermore, the three-dimensional substrate is excellent in mechanical characteristics such as tensile strength, elongation percentage, etc., and these characteristics are important to roll the electrode stably.
However, when a paste-type electrode containing the felt-like metal porous member and having one polarity, is rolled in a spiral manner along with another electrode having the other polarity and a separator interposed between the electrodes to assemble an alkali secondary cell, both electrodes are likely to touch with each other, thereby short-circuiting within the cell. More specifically, since the felt-type metal porous member is made of slender hollow metal fibers, each junction where metal fibers intersect with each other is small. Consequently, while rolling the paste-type electrode made of the felt-like metal porous member in a spiral manner as mentioned above, some of the hollow metal fibers of the porous member are broken off and cut from their junctions, and the ends of broken metal fibers are held pointed outward. As a result, the pointed ends of those broken fibers pierce the separator and are brought into contact with the other electrode having the opposite polarity, thereby short-circuiting within the cell.
Further, since the hollow metal fibers of the porous member are likely to be cut from junctions while rolling the paste-type electrode, transmission of electrons through the fibers is degraded, thereby lowering the porous member's collecting performance. Thus, the utilization of the activation substance is lowered.
In the meantime, U.S. Pat. No. 4,251,603 discloses a technique in which foaming resin is plated with metal and baked, and that in which foaming metal having a lattice diameter of the surface portion larger than that of the inner portion is used as a three-dimensional substrate.